User equipment (UE), also known as mobile stations, wireless terminals and/or mobile terminals are enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system. The communication may be made e.g. between two user equipment units, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks.
The user equipment units may further be referred to as mobile telephones, cellular telephones, laptops with wireless capability. The user equipment units in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another user equipment or a server.
The wireless communication system covers a geographical area which is divided into cell areas, with each cell area being served by a network node, or base station e.g. a Radio Base Station (RBS), which in some networks may be referred to as “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and terminology used. The network nodes may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the network node/base station at a base station site. One base station, situated on the base station site, may serve one or several cells. The network nodes communicate over the air interface operating on radio frequencies with the user equipment units within range of the respective network node.
In some radio access networks, several network nodes may be connected, e.g. by landlines or microwave, to a Radio Network Controller (RNC) e.g. in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Special Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), network nodes, or base stations, which may be referred to as eNodeBs or even eNBs, may be connected to a gateway e.g. a radio access gateway. The radio network controllers may be connected to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units. The 3 GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
The 3GPP is responsible for the standardization of UMTS and LTE. LTE is a technology for realizing high-speed packet-based communication that may reach high data rates both in the downlink and in the uplink, and is thought of as a next generation mobile communication system relative UMTS.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the network node to the user equipment. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction i.e. from the user equipment to the network node.
The basic principle used in cellular wireless communication systems (GSM, UMTS, LTE) comprises the following steps:                (a) each network node sends a pilot signal,        (b) the user equipment measures the received pilot signal strength from the serving network node and from several neighbouring network nodes,        (c) the user equipment reports back to the serving network node the result of the measurements, and        (d) the serving network node decides whether the user equipment should be handed over to another cell or not.        
Typically, the user equipment is instructed to connect to the network node with the highest received pilot power, which defines the reference cell size.
In order to reduce the amount of measurement reports sent to the user equipment to the network node, the user equipment is instructed to perform comparisons between the received pilot signal from the serving network node and from the neighbouring cells, and to report the measurements only if some predefined criterion is fulfilled. For instance, the user equipment may report measurements only if the signal from a neighbouring cell is 3 dB larger than the signal from the serving cell. Besides such thresholds, the criterion may also comprise timers, etc. for avoid toggling between cells.
The decision to handover the user equipment is nonetheless taken by the serving network node. Hence, even if the power from a neighbouring cell exceeds by far the power received from the serving cell, the serving network node may still decide not to handover the user equipment for other reasons such as e.g. load balancing.
All systems, like GSM, UMTS or LTE implement this type of mechanism.
The aforementioned procedure of selecting the cell with the strongest pilot signal is typically the best selection for the downlink, because high received pilot power typically corresponds to high SNR on the downlink. In LTE, the procedure is referred to a Reference Signal Received Power (RSRP) based cell selection.
If the transmitted pilot power is the same for all the network nodes, then equal received RSRP from two network nodes corresponds to equal pathloss to these two network nodes. In this case, the RSRP-based cell selection is optimal also on the uplink. However, if there is a difference between the pilot power sent by two network nodes, then the RSRP from the high-power network node may be stronger than the RSRP from the low-power network node, although the user equipment is closer to the low-power network node from a radio and geographic point of view, as the case may be in heterogeneous networks. The coverage of the low-power network node may be significantly smaller than the coverage of the high power network node.
It is well known that in such networks, also called Heterogeneous networks (HetNet), the RSRP cell selection is sub-optimal for the uplink. A known solution to this problem is to extend the coverage of the low-power network node by adding an offset to the RSRP measured from the low-power network node. The larger the cell selection offset, the larger the coverage of the low-power network node. If the pilot power difference between two network nodes is δ and the cell selection offset is set to δ, then basically the cell selection is based on the pathgain (coupling loss), which would be optimal for uplink. Thus the cell selection offset is configured to be between 0 and δ, so as to trade-off between the downlink and the uplink performances.
The standard for LTE-Advanced comprises relaying techniques which allow a network node to connect to another network node using the same radio interface and spectrum as for the connections with the user equipments. This allows the network node to connect to another network node within the wireless communication network when there is no alternative backhaul transport solution.
FIG. 1A presents a scenario in a heterogeneous network comprising a base station and a relay node. The base station and the relay node are connected via a wireless backhaul link. Also a user equipment is situated somewhere in between the base station and the relay node and may potentially be served by one of these nodes.
Assume that the user equipment in FIG. 1A receives a stronger RSRP from the relay node than from the base station. According to the standard procedure, the user equipment will get connected to the relay node. As previously discussed, the coverage of the relay node may be extended by using a positive cell selection offset δ.
Towards the base station, the relay node acts as a user equipment. Towards the user equipment, the relay node acts as a base station. The user equipment may not make any logical distinction between a cell created by a relay node and the cell created by a regular base station. In fact the user equipment may not even be aware of the existence of the radio backhaul connection.
Another relevant aspect of relaying networks is the spectrum resources used for the backhaul. One distinguishes between out-of-band and in-band relaying. In the former case, the backhaul link uses a different frequency carrier and even a different access technology than the access link uses. In this case, when using out-of-band relaying, the interference between the two links may be negligible and the two links may be managed independent on each other. In the later case, the backhaul and the access link use the same carrier and therefore may interfere with each other.
A typical limitation of in-band relays is that are not be able to send and receive on the same channel, at the same time, i.e., they use a half-duplex communication mode. In order to send and receive on the same channel, the relay node would need two antenna system (one for communicating with the donor base station, and one for communicating with the user equipments), and the isolation between the two antenna systems needs to be very high. This would increase the cost and the dimensions of the relay node, which for most of practical deployment cases would make the relay node unfeasible. Therefore in-band-relaying typically leads to increased interference and increased data-buffer lengths, causing increased delays within the network.
While cell extension through a handover offset is a possible solution to mitigate the interference problem in heterogeneous networks, and alternative solution may be for the (donor) base station to schedule those user equipment which are located close to the relay node on the same type of resources as the backhaul link. By doing so, the user equipment will not be active at the same time as the access links connected to the relay node, and therefore the user equipment and the relay node will not cause interference to each other.
As the subsequent disclosure exclusively relates to half-duplex in-band relay nodes, the term in-band is omitted in the subsequent text. By “relaying” it is herein meant “in-band relaying”, unless otherwise stated.
A common problem when relaying in half-duplex is that effective data-rate performance for some user equipment may actually be degraded when using relay nodes.
Assume that the communication between source and destination may use only one resource element, for instance a time slot. The communication may be either direct over the radio link denoted Gd in FIG. 1A i.e., the user equipment selects the base station cell, or through a relay node i.e., the user equipment selects the relay cell.
FIG. 1B shows the resource allocation in a prior art relaying network. On the Y axis, the resources indicated between time slot index 0 and n are used by the relay node only for the backhaul communication. The resources n+1 to 10 are utilized for the signalling to/from user equipment. Thus interference between the backhaul and the signalling made via the relay node to/from user equipment within the cell of the relay node is avoided. On the X-axis is, in a schematic way, the distances between the base station and the relay node, in a radio sense, and also the estimated cell limit between the respective cells indicated.
The resources 0 to n may also be used by for the communication between the base station and the user equipment connected to the base station. However, the base station is using a time slot (or a radio resource in general) to communicate either with the relay, or with a user equipment, but not with both at the same time.
However, the limitation of half duplex transmission over the backhaul link also leads to a situation where the end-to-end data rate performance decrease when transmitting via the relay node, in comparison to direct transmission from the base station, as the resources have to be shared between the backhaul link and the link between the relay node and the user equipment. There will also be a delay in the relay node when shifting from receiving signals from the base station over the backhaul link and transmitting signals to the user equipment within the own cell, leading to a degradation of performance within the wireless communication system.
Thus, it would in many cases be an advantage from this point of view, to let user equipment connect directly to the base station, even if they are situated close to the relay node. However, if that is done, interference would be generated between the user equipment and the relay node for user equipment situated close to the relay node.
More exactly, the relay node would generate huge downlink interference to the user equipment, while the user equipment would generate huge uplink interference to the relay node, which would render such cell selection method impossible to use.
In a low-cost deployment of relay nodes, it is likely for some user equipment to be located close to the relay node, although they have good direct link to the base station. Previous studies have shown that this leads to severe degradation of the network performance, in the sense that many user equipment units that expected to have high data rates by connecting directly to the base station, instead experience poor data-rates when the relay nodes are added.
If this problem is not solved, it is likely for the operators and end-users to be very unhappy with the introduction of relay nodes.